--- /dev/null
+(**************************************************************************)
+(* ___ *)
+(* ||M|| *)
+(* ||A|| A project by Andrea Asperti *)
+(* ||T|| *)
+(* ||I|| Developers: *)
+(* ||T|| The HELM team. *)
+(* ||A|| http://helm.cs.unibo.it *)
+(* \ / *)
+(* \ / This file is distributed under the terms of the *)
+(* v GNU General Public License Version 2 *)
+(* *)
+(**************************************************************************)
+
+include "static_2/notation/relations/positive_3.ma".
+include "static_2/syntax/item_sd.ma".
+include "static_2/syntax/term.ma".
+
+(* DEGREE POSITIVITY ON TERMS ***********************************************)
+
+inductive tdpos (h) (o): predicate term ≝
+| tdpos_sort: ∀s,d. deg h o s (↑d) → tdpos h o (⋆s)
+| tdpos_lref: ∀i. tdpos h o (#i)
+| tdpos_gref: ∀l. tdpos h o (§l)
+| tdpos_pair: ∀I,V,T. tdpos h o V → tdpos h o T → tdpos h o (②{I}V.T)
+.
+
+interpretation
+ "context-free degree positivity (term)"
+ 'Positive h o T = (tdpos h o T).
+
+(* Basic inversion lemmas ***************************************************)
+
+fact tdpos_inv_sort_aux (h) (o):
+ ∀X. 𝐏[h,o]⦃X⦄ → ∀s. X = ⋆s → ∃d. deg h o s (↑d).
+#h #o #H *
+[ #s #d #Hsd #x #H destruct /2 width=2 by ex_intro/
+| #i #x #H destruct
+| #l #x #H destruct
+| #I #V #T #_ #_ #x #H destruct
+]
+qed-.
+
+lemma tdpos_inv_sort (h) (o): ∀s. 𝐏[h,o]⦃⋆s⦄ → ∃d. deg h o s (↑d).
+/2 width=3 by tdpos_inv_sort_aux/ qed-.
+
+fact tdpos_inv_pair_aux (h) (o): ∀X. 𝐏[h,o]⦃X⦄ → ∀I,V,T. X = ②{I}V.T →
+ ∧∧ 𝐏[h,o]⦃V⦄ & 𝐏[h,o]⦃T⦄.
+#h #o #H *
+[ #s #d #_ #Z #X1 #X2 #H destruct
+| #i #Z #X1 #X2 #H destruct
+| #l #Z #X1 #X2 #H destruct
+| #I #V #T #HV #HT #Z #X1 #X2 #H destruct /2 width=1 by conj/
+]
+qed-.
+
+lemma tdpos_inv_pair (h) (o): ∀I,V,T. 𝐏[h,o]⦃②{I}V.T⦄ →
+ ∧∧ 𝐏[h,o]⦃V⦄ & 𝐏[h,o]⦃T⦄.
+/2 width=4 by tdpos_inv_pair_aux/ qed-.
+
+(* Basic properties *********************************************************)
+
+axiom tdpos_total (h): ∀T. ∃o. 𝐏[h,o]⦃T⦄.